Comparison of DC-link Voltage Balancing Strategies for Three Level NPC Inverter

A major drawback of three-level neutral point clamped (NPC) inverters is the imbalance in the DC-link voltage. The unbalanced capacitor voltage fluctuates the neutral point voltage and causes low-frequency oscillations subsequently. Therefore, balancing the dc-link capacitor voltage is essential for NPC inverters in order to minimize the neutral point fluctuations and oscillations. This paper reviews two dc-link voltage balancing strategies, viz., the carrier-based pulse width modulation (CBPWM) and discontinuous pulse width modulation (DPWM), in terms of balancing capacitor voltage. A detailed theoretical analysis is performed on the select strategies and the performance analysis is done by simulation studies.